But it was your letter within, dear heart, my first love letter, and sweeter to me than the subtlest love-lyric Sappho ever penned in Aeolic gold.

The speaker recalls receiving a beloved’s first love letter and elevates it above even the most refined tradition of erotic lyric poetry. By invoking Sappho—icon of intimate, musically wrought love verse—and the phrase “Aeolic gold” (a nod to Sappho’s Aeolic Greek and to the preciousness of her art), the line contrasts literary sophistication with the immediacy of personal feeling. The “letter within” suggests an enclosed note discovered inside another object (perhaps a book or parcel), emphasizing surprise and privacy. The passage ultimately argues that authentic, lived affection can surpass canonical art in sweetness and value, because it is addressed to the self and bound to memory.